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The Power-Saving Power Factor

How to minimize electrical losses and make power companies happy

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Typically, a power company’s goal is to sell as much electricity as it can. But power companies don’t like to "make" more electricity to compensate for power that’s "lost" on the way from a power station to an electrical meter, which refers to losses in the electrical distribution system.

This month’s topic, saving power by reducing losses in neon circuits, requires some discussion about alternating-current (AC) theory. However, for more in-depth information, readers should consult electrical-engineering books.

"Power factor," the ratio between the actual load power and apparent load power, indicates how effectively a current is converted into useful work output.

Every wire has a resistance — the longer and thinner the wire, the higher its resistance. And, every current that passes through a resistance creates losses. For a given wire length, resistance and losses can be reduced by increasing the wire diameter, which results in increased material costs. A power company’s primary goal is to distribute power and, at the same time, incur fewer losses and minimal distribution-center costs.

Voltage, current and power

Electrical power is defined by the actual voltage at the load, multiplied by the current flowing through this load. With direct current (DC), everything is constant in time, and the measurement of current and voltage (and multiplication to determine the power drawn) is quite simple. But, in nearly all U.S. towns, AC networks are used today.

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With AC, the voltage varies with time from zero through positive maximum, through zero again, through a negative maximum, back to zero again, and then repeats. In the United States, AC reverses direction 120 times every second. But what happens to the current when the voltage alternates?

With a resistance load on an alternating current source, the current rises every time the voltage rises, and vice versa. Or, simply, the current tracks the voltage.

To determine how much power is drawn, multiply the current by the voltage — at every point on the curve — and sum it up over a complete cycle (this is what engineers call "integrating over time"). Now, you’d expect the result to be zero, implying that positive and negative parts have the same value and would cancel each other out.

However, this isn’t the case. Negative voltage multiplied by negative current renders a positive result, so the total power is positive. Consequently, the electrical meter will run. $image1

Capacitors and inductors on AC

Neon signs can’t be powered directly from the main supply because the voltage is insufficient for neon tubes. Thus, we need transformers to step up the voltage and, at the same time, limit the current (see ST, June 1999, page 22). This makes neon transformers different than normal distribution transformers.

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Every inductor (copper coil on iron core) behaves like a heavy mass, meaning, it will try to stay in the present state and avoid any change (in electrical terms). So when a voltage is applied to an inductor momentarily (by switching on the supply), the transition from no current to current flow takes time.

This is due to the so-called "self-induction." In other words, the current in an inductor rises slowly after voltage is applied, because it takes time to reach its maximum due to the inductive inertia. The bigger the inductance, the more time it takes. The principle of inductance is analogous to a mechanical flywheel.

Regarding AC waveforms, when we place an inductive load, instead of a lightbulb, into a circuit, the waveform will look like Fig. 1. In Fig. 1, when we multiply the current and voltage, the current is at its maximum when there’s no voltage, and vice versa. Also, at certain times, we’ll have a positive current at a negative voltage, thus making negative power. Negative power? Yes.

Negative power means that our inductive load (the neon transformer) is feeding power back into the main line. If I calculate the power in Fig. 1 correctly, over time, the sum of all positive and negative power levels will cancel each other out, making the total power consumption zero.

Zero? Yes. If I have a completely inductive load, the electrical meter won’t run at all because there’s no "effective power" consumed. To electricians, this means the "power factor" of a pure inductive load is zero.

Indeed, we have current and voltage in our main line, but not at the same time, in the same direction. As stated earlier, the current in the line wires causes losses, for which power companies must compensate. This is why power companies don’t like the "reactive power" of inductive loads.

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In reality, we don’t have a completely inductive load as seen in Fig. 1. The real neon transformer load is always a mix of Ohmic and inductive load. If you carefully read a typical neon transformer’s label, you’ll notice several numbers: the primary amperes, the primary volts, and either a number "power factor" or "cos w," or a consumption in watts.

By multiplying the given primary volts and amps, we achieve the so-called "apparent power" rating. Thus, we’d measure voltage and current separately as average value and then multiply them. (Doing so in Fig. 1 will generate a high positive value, even if the power meter doesn’t run.) The real (or effective) power consumed (the power the meter will count) is the apparent power multiplied by the power factor.

For a typical neon transformer, the power factor is between 0.3 and 0.5; thus, the apparent power is more than double the real, or effective, power. Consequently, the wiring the power company provides to the site, as well as the wiring in the building, has to be sized according to the apparent power (see the National Electric Code® [NEC], Article 310 and ST, July 2002, page 64), which is more than twice than what would be needed at the optimum power factor (i.e. cos w=1).

This is why power companies charge extra if a site’s apparent power consumption is higher than the effective power. However, it’s incorrect to say the apparent power is real consumption, which is a common misconception some companies use to incorrectly compare LED and neon efficiency. This claim suggests a higher consumption for neon than is realistically true.

The way out — compensation

The losses in the lines could be reduced if there’s a way to shift the current to make it occur at the same time as the voltage. A capacitor load placed in an AC circuit shows the property that the current will lead the voltage — the exact opposite characteristic of an inductive load (Fig.2).$image2

Just imagine combining the correct amount of capacitance (current leading) with the inductance from our neon transformer (current lagging) so the total current is neither lagging nor leading. The result: a circuit where the apparent power equals the effective power, and the wires and distribution network don’t carry additional inductive currents.

Keep in mind, when we have pure effective power in our circuit, the power factor is 1, which is high compared to a "normal power factor" neon transformer of approximately 0.3 to 0.5. Thus, neon transformers with this integrated capacitor are called "high-power-factor transformers."

Also, it’s wise to add these capacitors when using standard neon transformers to increase the power factor. In Europe, power companies require a minimum power factor of 0.9 for signs with a load of more than 2,500W. The calculation required to achieve the correct capacitor value is too detailed to include here.

Therefore, always ask transformer manufacturers to recommend the correct values. Remember, by increasing the power factor of a neon sign, your clients and the environment will profit.

Power Factor Dos and Don’ts
*Don’t use high-power-factor transformers or compensation capacitors in dimmed neon installations. They will damage the dimmer.
*Don’t place high-power-factor transformers or compensation capacitors behind a flasher when neon flashes rapidly. Instead, install a separate capacitor before the flasher.
*Don’t install electronic neon power supplies with compensation capacitors. Both will be damaged if you do so.
*Don’t add capacitors to the neon transformers where supposed "central compensation" is employed in a building. It’s unnecessary.
*Do rate compensation capacitors. They should be rated for the same temperature rate as the transformer.
*Do install compensation capacitors as closely as possible to the neon transformer in the transformer box.
*Do use "high-power-factor" transformers with built-in capacitors (where possible). They’re listed as perfect matches for the transformer.
*Do talk to the power company about power-factor requirements for every sign installation with a total load of more than 5,000W.

 

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